1. Field of the Invention
The present invention relates to a sheet feeder and an image forming apparatuses including the sheet feeder.
2. Description of the Related Art
A sheet feeder provided in an image forming apparatus to feed paper one sheet by one sheet from a paper tray where a plurality of sheets of the paper for use in image formation is stacked is known. The sheet feeder includes a feed roller that conveys the sheet of paper picked up from the paper tray and a conveying roller (also referred to as a “grip roller”) that conveys the paper conveyed by the feed roller downstream in a conveying direction. A two-circuit drive system made up of a drive system for driving the feed roller and a drive system for driving the conveying roller is necessary to drive the sheet feeder.
Such a two-circuit drive system can be constructed using a structure in which the feed roller and the conveying roller are driven by different driving sources. However, this structure requires that a single paper tray should include at least two driving sources. Accordingly, the number of driving sources becomes large in an image forming apparatus that includes a plurality of paper trays, which disadvantageously leads to an increase in cost and an increase in space occupied by the drive system. To that end, there is proposed a technique of providing a single driving source in a single paper tray and driving a feed roller and a conveying roller by switching a rotating direction of the driving source in the middle of paper conveyance.
Known examples of a sheet feeder that employs such a driving motor of which rotating direction can be reversed include that disclosed in Japanese Patent Application Laid-open No. 2008-239343 and that disclosed in Japanese Patent No. 3782721.
According to a technique disclosed in Japanese Patent Application Laid-open No. 2008-239343, rotations of the sheet feed roller and the conveying roller are controlled using a motor of which rotating direction can be reversed. This control is performed such that when the motor rotates forward, the sheet feed roller and the conveying roller are rotated, but when the motor rotates backward, only the conveying roller is rotated. More specifically, the motor is rotated forward when sheet feeding is started, and stopped when a sensor detects that a leading end of the sheet (paper) has reached a nip portion between registration rollers. After a lapse of a predetermined period of time, the motor is rotated backward in synchronization with rotations of the registration rollers to thereby rotate only the conveying roller.
When a sheet is to be fed from a second (lower) paper cassette of vertically-stacked two paper cassettes, a first motor of a first (upper) paper cassette is rotated backward, and a second motor of the second paper cassette is rotated forward until the sheet reaches the nip portion. When the sheet reaches the nip portion, the second motor is rotated backward as in the case described above, causing both the first motor and the second motor to rotate backward. When a sheet of a small size is fed, the second motor is temporarily stopped, and thereafter rotated backward to prevent feeding of multiple sheets (double feed of sheets).
According to a technique described in Japanese Patent No. 3782721, a sheet feeder rotates a feed roller, a separation roller, and a conveying roller using a single motor of which rotating direction can be reversed. More specifically, when the motor rotates forward, all of the feed roller, the separation roller, and the conveying roller are rotated, but when the motor rotates backward, only the separation roller and the conveying roller are rotated. Switching between the forward rotation and the backward rotation of the motor takes place when a sensor arranged downstream of the conveying roller detects a leading end of paper.
When a plurality of sheet feeders are provided, switching may be performed as follows, for example. A motor of an upper sheet feeding unit is rotated backward, and when a sheet is to be fed from a lowermost sheet feeding unit, rotation of a motor of the lowermost sheet feeding unit, from which the sheet is to be fed, is switched from forward rotation to backward rotation in the middle of sheet conveyance.
To switch the rotating direction of the driving motor in the middle of sheet feeding as described above, it is necessary to decelerate the driving motor to a halt, reverse the rotating direction, and thereafter accelerate the motor. Time is lost in the process of this series of operations. In addition, backlash of meshed gears further increases the loss of time. These set a limit on further increase in processing speed. Furthermore, abrupt reversing of the rotating direction can disadvantageously increase a load placed on the drive system.